On Sep. 8, 1996 a propane transport tanker filled with 9,000 gallons of liquid propane was delivering product to a bulk storage tank system. During the delivery, the line used to transfer the liquid propane from the transport trailer to the storage tanks failed. This allowed liquid propane to escape in an uncontrolled manor. The existing safety valve, called an excess flow valve, located at the discharge port of the MC330/MC331 class tanker, failed to close because not enough differential pressure between the transport tank vessel and the discharge line directly downstream of the excess flow valve existed to close the valve. Propane fluid flowed past the excess flow valve, and across the product pump pumping mechanism into the severed line.
Because propane fluid expands at a rate of approximately 400% when it reaches the atmosphere, the severed hose moved in a fast and uncontrollable motion. The transport truck and trailer were parked in such a way that the operator could not reach the manual shutoff valves located at opposite ends of the trailer which would have closed the discharge valve on the transport vessel.
A check valve designed to prevent liquid propane from escaping from the storage tank's inlet line also failed. This failure allowed approximately 30,000 gallons of liquid propane to escape from the propane storage facility.
After an investigation by the United States Department of Transportation (D.O.T.) into the accident, it was determined that existing systems for shutting off product flow in case of a line failure do not always operate properly. On Aug. 15, 1997 the Research and Special Programs Administration (RSPA) of the U.S. Department of Transportation issued a final ruling under 49 C.F.R. .sctn.171.5 of the Hazardous Materials Regulations requiring all MC330/331 (a classification of the United States Department of Transportation) transport vessels to install a working device to detect a failure in the pressure line between the transport vessel and the receiving storage vessel and automatically shut off the flow of product from the transport tank without human interaction or assistance.
Existing manual safety shutdown valves are typically already in place on most of the MC330/MC331 tankers in operation. They primarily consist of manual valves with a self closing spring and a latch held in the open position by a pin connected to a cable. The cable is in turn connected to a spring chamber, which is held extended with air pressure. If a condition arises in which the flow of product must be stopped, the operator must activate one of the manual air valves located at opposite ends of the trailer which will vent the spring chamber which with the lack of air pressure will retract and pull on the cable. This withdraws the pin and closes the latch, which closes the fluid product shutoff valves connected to it. If a fire were to start the air lines, which would melt very quickly, venting the spring chamber, and closing the product valves as well, without human intervention.
Existing shutdown systems, as described above, do not meet the requirements required by the above D.O.T. mandate because they do not operate reliably under all pressure and temperature conditions or they require human intervention to activate the safety device. The existing automatic excess flow shutoff valve located at the outlet port or ports of the tanks on an MC330/MC331 Transport or Bobtail work well only in the unlikely event that the attached plumbing between the outlet of the excess flow valve and the product pump is sheared off in an accident. This shearing causes a high differential pressure across the excess flow that valve would that usually force the excess flow valve to close.
In contrast, during normal unloading of product the excess flow valve must be set for a higher flow rate than the product pump transfers product out of the transport vessel or the valve would close during normal pumping operations. Because a line break typically occurs in the flexible line located between the pump outlet and the receiving stationary vessel, the actual flow rate across the excess flow valve tends to remain about the same because of the presence of the product pump in the line which tends to act as a line restriction. This allows product to continue to escape even if the product pump has stopped turning because a higher than usual flow condition is not created which would force the excess flow valve to close.
The industry has known for decades that existing excess flow valves did not operate under all known temperature and operating conditions. Although accidents and injuries are not common, the potential for catastrophic property damage, injury, and death does exist if a propane or butane leak were to ignite, or Anhydrous Ammonia (Nh3) were to drift into a populated area. The existing manual shutdown systems currently installed on all MC330/MC331 transport vessels in addition to the excess flow valves, must be activated by the operator if a problem should arise.
The pressure required to transfer liquid propane, Anhydrous Ammonia (Nh3), and Butane vary depending on line size, product temperature, air temperature, and the pressure existing in the receiving tank system. The industry needs a device that will detect a break in the line between the transport vessel and the receiving vessel during unloading operations and automatically close all valves allowing product to escape from the transport vessel. In addition to this requirement the device must also automatically shutoff the flow of product from the vessel in case of a fire. A fusible link or device must cause the product valves to close when the temperature exceeds 250.degree. Fahrenheit. Further, the device must be compatible with existing equipment already in the field and use as much of the existing equipment on each installation as possible to reduce installation expenses and operator inconvenience.